Ballasting is a standard shipping practice. Upon departure from a port, ballast water is pumped into ballast tanks located in the hull of the ship. The ballast water provides additional weight to increase the stability and maneuverability of ships at sea. Ballast water is later discharged just before reaching the destination port to reduce the vessel's draft, allowing the ship to navigate in shallower water.
Though a common practice, ballasting has significant negative environmental consequences. During the loading of ballast water at the port of origin, pollutants and marine organisms present in the water column and harbor floor can be sucked into the ballast tanks of ships. Upon deballasting, these marine organisms and pollutants are released at the ship's destination.
Ballast water has been shown to harbor chemical pollutants, including volatile organic compounds (VOCs), as well as marine organisms ranging from microscopic organisms such as plankton, bacteria, and protozoans, to macroscopic organisms including shellfish and aquatic plant life. Aquatic invasive species (AIS) can be introduced into environments via ballast water with devastating environmental and economic consequences. The introduction of the zebra mussel into the Great Lakes of the United States is a well-publicized example of the environmental impact posed by AIS carried by ballast water.
Ballast water taken into a tank from one body of water and discharged in another body of water can introduce invasive species of aquatic life. The discharge of water from ballast tanks has been responsible for the introduction of species that cause environmental and economic damage. The International convention for the control and management of ships' ballast water and sediments in 2004 tries to deal with this environmental problem, by regulating the discharge and charge of ballast water. Ships must comply with the regulations in effect and change their ballast water when they sail in high seas. The exchange should be made by high depth (more than 2000 meters) so far as possible, during the day and as far away from the littoral zone as possible. To avoid stability or mechanical stress problems, there are two options for the replacement of ballast water: empty the whole tank and fill it, or without emptying the tank, proceed with the filling by allowing the overflow to come out through the air outflow, in which case three times the volume of the tank has to be exchanged.
To mitigate the potential impact of ballast water, ballast water can be treated before it is discharged. Existing methods for the remediation of ballast water are similar to methods used to treat water in other applications, and include chemical disinfection, ultraviolet (UV) irradiation, filtration, deoxygenation, microagitation, and electrochlorination. However, all of these existing methods possess significant drawbacks.
Chemical disinfection involves contacting ballast water with a chemical disinfectant such as chlorine, ozone, peracetic acid, or menadione, prior to discharge. Because the disinfected ballast water is ultimately expelled from the vessel, the chemical disinfectant added to the ballast water, as well as any byproducts formed during ballast water treatment, are also discharged. Absent additional remediation, chemically disinfected water can be destructive to ecosystems. In addition, chemical disinfection requires the storage of hazardous chemicals on board ships, which takes up valuable storage space and requires special handling and disposal methods, as well as presents risks to human health and the environment.
Ballast water can also be irradiated with ultraviolet (UV) light. However, this method requires the transmission of UV light through the ballast water. As a result, UV irradiation cannot effectively sterilize turbid water. In addition, UV irradiation of ballast water requires significant power consumption, and a large footprint in the case of systems requiring a high volumetric flow rate.
Deoxygenation involves removing all of the oxygen from ballast water prior to discharge in order to asphyxiate any marine organisms in the water. Deoxygenation is a lengthy process, typically requiring days to complete. In addition, water must be re-oxygenated prior to discharge. Finally, this method will not eliminate chemical pollutants, such as VOCs, or organisms that do not require oxygen to survive.
Microagitation involves the formation of microbubbles in a flowing liquid. The collapse of the microbubbles disrupts the cell walls of microorganisms, eliminating microorganisms present in the ballast water. However, microagitation is energy intensive, and requires careful engineering. In addition, microagitation will not eliminate chemical pollutants, such as VOCs, present in the ballast water.
In electrochlorination (also referred to as electrolytic disinfection), direct current is applied to the ballast water. Seawater contains dissolved sodium chloride. The applied electric current reacts with the dissolved sodium chloride to form sodium hypochlorite (i.e., bleach) which sterilizes the ballast water. This method is effective at neutralizing species in the ballast water and does not require the storage of hazardous chemicals. However, electrochlorination is energy intensive. Significant quantities of energy are required to form an effective concentration of bleach in ballast water. In addition, electrochlorination performance is reduced at temperatures between 10-15° C. [50-59° F.] and does not function at all below 5° C. [41° F.]. Therefore, additional energy is typically required to heat ballast water to above 15° C. [59° F.], a temperature range at which electrochlorination is efficient. Finally, electrochlorination systems typically discharge effluent containing bleach, which can be destructive to marine ecosystems.
Accordingly, there is a need for systems and devices for the simple, safe, energy efficient, cost-effective, and environmentally benign remediation of water, particularly ballast water.
It is therefore the object of the invention to provide improved systems for the remediation of water, including ballast water.
It is further an object of the invention to provide systems for the remediation of water, including ballast water, which can simultaneously eliminate chemical pollutants and biological contaminants.
It is also an object of the invention to provide systems for the remediation of water, including ballast water, which can effectively eliminate chemical pollutants and biological contaminants without generating intermediates or byproducts which require further remediation.
It is also an object of the invention to provide systems for the remediation of water, including ballast water, which do not require the addition of chemical reagents.